Friction stir grain refinement of structural members

ABSTRACT

The present invention provides an improved structural assembly constructed of a plurality of structural members secured together. At least one of the plurality of structural members defines a first region characterized by comparatively high operational stress and a second region having a locally refined grain structure positioned such that the second region at least partially encompasses the first region to thereby enhance the strength, toughness and fatigue resistance of the at least one structural member in the first region. The present invention also provides a method for selectively improving the strength, toughness and fatigue resistance of a structural member in a region of high operational stress including the steps of casting the structural member in a pre-selected configuration. Regions of the structural member having comparatively high operational stress are identified. The structural member is secured to prevent movement. The region of the structural member having comparatively high operational stress is then mixed with a rotating friction stir welding probe to locally refine the grain structure of the structural member within the region of high operational stress to thereby improve the strength, toughness and fatigue resistance of the structural member in the region. The structural member can be secured to other structural members to form the frame of an aircraft. The improved structural assembly will have an increased operational life, as well as require less stock material with a corresponding decrease in the overall weight of the assembly.

FIELD OF THE INVENTION

The present invention relates to selectively improving the materialproperties of structural members and, more particularly, relates toselectively refining the grain structure of structural members.

BACKGROUND OF THE INVENTION

Conventional structural assemblies, such as those used in themanufacture of military and commercial aircraft, are commonly fabricatedfrom a plurality of structural members secured together to form abuilt-up structure. The structural members are typically forged,machined from stock material or cast in various configurations fromsteel, stainless steel, magnesium, magnesium alloys, copper, copperalloys, brass, aluminum, aluminum alloys, or titanium.

During use, aircraft structural assemblies are subjected to static andcyclic loads, as well as a variety of environmental conditions,temperature variations, and severe acoustic and vibration environments,all of which create mechanical and thermal stresses. While theseoperational stresses generally exist throughout the individualstructural members forming the structural assembly, certain regions ofeach structural member are typically subjected to comparatively highermagnitudes of stress. For example, under cyclic loading conditions,threaded openings machined into a structural member to facilitateattachment to other structural members when forming a structuralassembly can significantly increase the stress in the immediate vicinityof the opening. High operational stresses can lead to micro-cracking orfracture of the structural members of a structural assembly, which canresult in the eventual failure of the assembly. In addition, due to thelarge number of parts and fasteners utilized in the construction ofconventional structural assemblies, maintenance, repair and replacementof structural members, if necessary, can be time consuming and laborintensive, which can be costly over the life of the assembly.

In seeking to enhance the strength, toughness and fatigue resistance ofstructural members and, thus, increase the useful life of structuralassemblies, designers have modified the dimensions of the structuralmembers in the regions of high operational stress, for example, byincreasing the thickness of the members in these regions. Designers havealso experimented with substituting more exotic and, typically, moreexpensive types of materials for use in the fabrication of thestructural members. Structural members can also undergo precipitationhardening whereby the members are solution heat treated and then aged atpredetermined temperature schedules to thereby improve the grainstructure and, thus, the material properties of the members. However,the precipitation hardening process can be time and labor intensive andprovides only limited improvement of material properties, and evenselective increases in the thickness of a structural member cannegatively increase the overall weight of the structural assembly, aswell as resulting in increased material cost.

Accordingly, there remains a need for improved structural members andmethods of manufacture that will increase the operational life ofstructural assemblies. The improved structural members must haveenhanced strength, toughness and fatigue resistance, especially in thoseregions subjected to high operational stresses.

SUMMARY OF THE INVENTION

The present invention provides a structural member defining a firstregion characterized by comparatively high operational stress and asecond region having a more refined grain structure than other portionsof the structural member positioned such that the second region at leastpartially encompasses the first region to thereby selectively improvethe strength, toughness and fatigue resistance of the structural memberin the first region. The structural member may be formed from steel,stainless steel, magnesium, magnesium-based alloys, brass, copper,beryllium, beryllium-copper alloys, aluminum, aluminum-based alloys,aluminum-zinc alloys, aluminum-copper alloys, aluminum-lithium alloys,or titanium.

The second region can be defined based upon the particular region thatwill be subjected to comparatively high operational stress. For example,the structural member may define a threaded opening at least partiallycontained within the second region. Alternatively, the structural membercan have an I-shaped configuration having opposed end portions and a webinterconnecting the end portions, wherein the second region encompassesat least a portion of the web of the I-shaped member. In anotherembodiment, the structural member has an I-shaped configuration whereinsaid second region includes at least a portion of at least one of saidopposed end portions. In yet another embodiment, the structural memberhas a tubular configuration. In still another embodiment, the structuralmember defines a plurality of regions having refined grain structures,wherein the regions are spaced apart and generally parallel. In stillanother embodiment, the structural member defines a first set of regionshaving refined grain structures and a second set of regions havingrefined grain structures. The first set of regions are spaced apart andgenerally parallel. The second set of regions are spaced apart andgenerally parallel and wherein the first set of regions intersects thesecond set of regions to thereby define a plurality of containmentzones.

The present invention provides a structural assembly including aplurality of structural members. The plurality of structural members aresecured together to form the structural assembly. The structural membersmay be formed from steel, stainless steel, magnesium, magnesium-basedalloys, brass, copper, beryllium, beryllium-copper alloys, aluminum,aluminum-based alloys, aluminum-zinc alloys, aluminum-copper alloys,aluminum-lithium alloys, or titanium. At least one of the plurality ofstructural members defines a first region characterized by comparativelyhigh operational stress and a second region having a more refined grainstructure than other portions of the structural member positioned suchthat the second region at least partially encompasses the first regionto thereby selectively improve the strength, toughness and fatigueresistance of the at least one structural member in the first region.

The second region can be defined based upon the particular region thatwill be subjected to comparatively high operational stress. For example,the at least one structural member may define a threaded opening atleast partially contained within the second region. Alternatively, theat least one structural member can have an I-shaped configuration havingopposed end portions and a web interconnecting the end portions, whereinthe second region encompasses at least a portion of the web of theI-shaped member. In another embodiment, the structural assembly has anI-shaped configuration wherein said second region includes at least aportion of at least one of said opposed end portions. In yet anotherembodiment, the at least one structural member has a tubularconfiguration. In still another embodiment, the at least one structuralmember defines a plurality of regions having refined grain structures,wherein the regions are spaced apart and generally parallel. In stillanother embodiment, the at least one structural member defines a firstset of regions having refined grain structures and a second set ofregions having refined grain structures. The first set of regions arespaced apart and generally parallel. The second set of regions arespaced apart and generally parallel and wherein the first set of regionsintersects the second set of regions to thereby define a plurality ofcontainment zones.

The present invention also provides a method for selectively improvingthe strength, toughness and fatigue resistance of a structural member ina region of high operational stress. According to one embodiment, themethod includes casting the structural member in a pre-selectedconfiguration. Alternatively, the structural member can be forged orfabricated as a wrought or machined part. Regions of the structuralmember having a comparatively high operational stress are identified.The structural member is secured to prevent movement. A region of thestructural member having comparatively high operational stress is thenmixed with a rotating friction stir welding probe to locally refine thegrain structure of the structural member within the region of highoperational stress to thereby improve the strength, toughness andfatigue resistance of the structural member within the region. Themixing step can include positioning a friction stir welding probeadjacent the region of the structural member having comparatively highoperational stress. A rotating friction stir welding probe can then beinserted through the outer surface of the structural member proximate tothe region of high operational stress to locally refine the grainstructure of the high-stress region. The rotating friction stir weldingprobe can be moved through the structural member along a pathcorresponding to the region of high operational stress. The frictionstir welding probe can be withdrawn from the outer surface of thestructural member to thereby define a threaded opening at leastpartially within the region of the structural member having a locallyrefined grain structure. If desired, the structural member can beprecipitation hardened prior to or after the inserting step.

After mixing the region of the structural member having thecomparatively high operational stress, the structural member can bemachined to a corresponding pre-selected shape and thickness. A threadedopening can be machined at least partially within the portion of thestructural member having a locally refined grain structure. Thestructural member can then be secured to other structural members toform the frame of an aircraft.

Accordingly, the present invention provides an improved structuralassembly and associated method of manufacture in which the assembly isconstructed from structural members having enhanced strength, toughnessand fatigue resistance in those regions subjected to comparatively highoperational stresses. The improved structural assembly will have anincreased operational life, as well as require less stock material witha corresponding decrease in the overall weight of the assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed in color.Copies of this patent with color drawing(s) will be provided by thePatent and Trademark Office upon request and payment of the necessaryfee.

The foregoing and other advantages and features of the invention, andthe manner in which the same are accomplished, will become more readilyapparent upon consideration of the following detailed description of theinvention taken in conjunction with the accompanying drawings, whichillustrate preferred and exemplary embodiments, and wherein:

FIG. 1 is a perspective view illustrating a structural assembly,according to one embodiment of the present invention;

FIGS. 2A, 2B, 2C, and 2D are cross-sectional views illustrating otherexemplary embodiments of structural members according to the presentinvention;

FIG. 3A is a cross-sectional view illustrating a cast I-beam that hasbeen selectively reinforced, according to one embodiment of the presentinvention;

FIG. 3B is a cross-sectional view illustrating the finish machinedprofile of the I-beam of FIG. 3A;

FIGS. 4A-4B are cross-sectional views illustrating conventional I-beamsbeing subjected to an alternating load, as is known in the art;

FIG. 5A is a cross-sectional view illustrating selective grain structurerefinement of a structural member by mixing the entire thickness of themember, according to one embodiment of the present invention;

FIG. 5B is a cross-sectional view illustrating selective grain structurerefinement of a structural member by mixing a portion of the thicknessof the member, according to another embodiment of the present invention;

FIG. 5C is a cross-sectional view illustrating selective grain structurerefinement of a structural member by mixing the entire thickness of themember, according to still another embodiment of the present invention;

FIG. 6 is a perspective view illustrating selective grain structurerefinement of a structural member, according to one embodiment of thepresent invention;

FIG. 7A is a plan view illustrating one embodiment of a structuralmember according to the present invention having a plurality ofreinforcing ribs;

FIG. 7B is a perspective view illustrating another embodiment of astructural member according to the present invention having a pluralityof containment zones;

FIG. 7C is a plan view illustrating another embodiment of a structuralmember according to the present invention having an open curvilinearcontainment zone;

FIG. 8 is a photograph illustrating the propagation of cracks along theperiphery of a region of locally refined grain structure, according toone embodiment of the present invention;

FIG. 9A is a plan view illustrating one embodiment of a structuralmember according to the present invention having a continuous area oflocally refined grain structure defined by a plurality of overlappingelongate regions of locally refined grain structure;

FIG. 9B is a cross-sectional view along lines 9B—9B of FIG. 9A of thestructural member of FIG. 9A;

FIG. 9C is a plan view illustrating the finish machined profile of thestructural member of FIG. 9A;

FIG. 9D is a cross-sectional view along lines 9D—9D of FIG. 9C of thestructural member of FIG. 9C;

FIG. 10 is a cross-sectional view illustrating one embodiment of astructural member according to the present invention having a threadedopening machined therein;

FIG. 11 is a plan view illustrating one embodiment of a structuralmember according to the present invention having a window;

FIG. 12 is a perspective view illustrating one embodiment of a tubularstructural member according to the present invention having spirallyconfigured regions of locally refined grain structure;

FIG. 13A is a perspective view illustrating one embodiment of a caststructural member according to the present invention prior to beingshaped into a finished configuration;

FIG. 13B is a perspective view illustrating the structural member ofFIG. 13A after being shaped into its finished configuration;

FIG. 14A is a cross-sectional view illustrating a cast structural memberhaving a recess machined therein;

FIG. 14B is a cross-sectional view illustrating the structural member ofFIG. 14A having an insert positioned within the aperture;

FIG. 14C is a plan view illustrating the insert joined to the structuralmember of FIG. 14B through a weld joint and a region of locally refinedgrain structure adjacent to the weld joint, according to one embodimentof the present invention;

FIG. 15 is a plan view illustrating an insert joined through a weldjoint to a structural member and a region of locally refined grainstructure adjacent to the weld joint, according to another embodiment ofthe present invention; and

FIG. 16 is a flow chart illustrating the operations performed, accordingto one embodiment of the present invention, in order to fabricate thestructural assembly of FIG. 1 and the structural members of FIGS. 2-15.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Referring to the drawings and, in particular, to FIG. 1, there isillustrated a structural assembly 10 according to the present invention.The structural assembly 10 can be used in the construction of a varietyof structures, including the frame of an aircraft. The structuralassembly 10 is constructed of a plurality of structural members 11 thatare welded together or secured using suitable fasteners 12. Thestructural assembly 10 illustrated in FIG. 1 includes two I-beams 14 andone planar member 15. However, other types of structural assemblies canbe constructed, if so desired. Although a variety of materials can beutilized, the structural members 11 are preferably formed from steel,stainless steel, magnesium, magnesium-based alloys generally, brass,copper, beryllium, beryllium-copper alloys, aluminum, aluminum-basedalloys generally, aluminum-zinc alloys, aluminum-copper alloys,aluminum-lithium alloys, or titanium. The structural members 11 can bemachined from stock material or cast. As illustrated in FIG. 2A-2D, thestructural members 11 can be cast or machined in a variety ofconfigurations, as is known in the art, based upon the load requirementsand other design parameters of the structural assembly 10.

As discussed above, during use, structural assemblies 10 are subjectedto static and cyclic loads, as well as a variety of environmentalconditions, temperature variations, and severe acoustic and vibrationenvironments, all of which create mechanical and thermal stresses, whichare collectively referred to herein as “operational stresses”. While theentire structural member 11 is generally subjected to varying magnitudesof operational stress, certain regions of each structural member aretypically subjected to comparatively higher magnitudes of operationalstress. For example, referring to FIGS. 4A and 4B , there is illustratedan I-beam 24 having opposed end portions 25 a, b and a web 27interconnecting the end portions. One of the end portions 25 a of theI-beam 24 is secured to a fixed support structure 29 while the other endportion 25 b is subjected to cyclic loading, such as an alternating load13 in directions 13 a, b perpendicular to the plane of the web 27, asillustrated in FIG. 4A, or an alternating bending load 13 c, asillustrated in FIG. 4B. The cyclic loading creates moment loads wherethe web 27 interconnects with the end portions 25 a, b. The moment loadsresult in bending and shear forces, which, when combined with the notcheffect of the sharp machined radii 28 at the interconnection of the weband the end portions, generally results in segments of the end portions25 a, b and the web 27 adjacent where the web interconnects with the endportions being subjected to comparatively higher magnitudes ofoperational stress. In another embodiment (not shown), the loads appliedto an I-beam can be such that regions of comparatively high operationalstress are located along the length of the web 27. Regions ofcomparatively high operational stresses can be identified from knownmathematical equations and computational techniques, such as finiteelement analysis.

As illustrated in FIG. 1, at least one structural member 11 of thestructural assembly 10 defines one or more regions 16 having a locallyrefined grain structure, which regions 16 at least partially encompass aregion of comparatively high operational stress. Preferably, the grainsize within the locally refined regions 16 ranges in order of magnitudefrom approximately 0.0001 to 0.0002 inches (approximately 3 to 5microns) and the grains have an equiaxed shape throughout the locallyrefined region. In contrast, while the grain structure of structuralmembers 11 formed by casting varies in size, shape and orientationdepending upon the composition of the cast alloy and the method ofcooling the structural member after being cast, typically, the grainsize of cast structural members ranges in order of magnitude fromapproximately 0.1 to 0.01 inches (approximately 2.5 to 0.25 mm). Sincethe edges of a cast structural member cool more quickly than theinterior portions of the member, the grains of a cast structural memberusually have a columnar shape at the edges of the member, transitioningto a cubic shape towards the interior of the member.

As illustrated in FIG. 6, the regions 16 of locally refined grainstructure are formed by mixing or stirring a portion of the structuralmember 11 with a non-consumable rotating friction stir welding probe 18.To effect mixing, the structural member 11 is first secured to aworktable of a friction stir welding machine by means of a conventionalclamp (not shown). The rotating friction stir welding probe 18 is theninserted through the outer surface 19 of the structural member 11 to apre-selected depth. An opening can be predrilled or tapped through theouter surface of the structural member 11 to receive the rotating probe,but preferably the rotating probe is thrust directly into and throughthe outer surface 19 of the structural member 11. Once inserted into thestructural member, the rotating probe 18 imparts mixing under theshoulder 20 of the probe by shearing action parallel to the outersurface 19 of the structural member. The rotating probe 18 also impartsmixing around the threaded portion 22 of the probe parallel to the probeaxis 23. See U.S. Pat. No. 5,460,317 to Thomas et al. for a generaldiscussion of friction stir welding, the entire contents of which areincorporated herein by reference.

The depth the rotating probe 18 is inserted into the structural member11 depends upon the material properties and dimensions of the structuralmember, as well as the types of loads that will be supported by, and themagnitude of operational stress that will be applied to, the structuralmember. Cyclic or alternating loads can result in micro-cracksinitiating on the surface of a structural member in the regions ofcomparatively high operational stress, which cracks can eventuallyresult in fatigue failure of the structural member. In one embodiment,only a portion of the thickness of the structural member 11, such as thewebs 17 of the I-beams 14 illustrated in FIG. 1 and the surfaces of thestructural members 11 illustrated in FIGS. 2A-2D, is selectively mixedto form regions 16 of locally refined grain structure (referred toherein as “partial-penetration mixing”). For example, topartial-penetration mix a structural member having a thickness of 1inch, the rotating probe can be inserted through the outer surface 19 ofthe structural member to a depth of approximately 0.25 inches.Advantageously, the regions 16 of locally refined grain structure resistthe formation and propagation of micro-cracks thereby selectivelyimproving the strength, toughness and fatigue resistance of thestructural member 11 in the corresponding regions of comparatively highoperational stress.

In another embodiment, as illustrated in FIG. 5B, the probe is thrust toa depth D where the tip 18 a of the rotating probe is a distance d fromthe opposite side 19 a of the structural member 11. During mixing, therotating probe 18 exerts approximately 1000 to 10,000 pound-force ormore on the structural member 11, depending on the size of the probe andthe depth of probe penetration, and generates sufficient frictional heatto raise the temperature of the portions of the structural memberadjacent the rotating probe to between approximately 700° F. and atemperature just below the solidus of the alloy forming the structuralmember. The frictional heat generated by the rotating probe 18 incombination with the force exerted by the probe on the structural membercan result in a forging-like effect on the unmixed portion 21 of thestructural member between the probe tip 18 a and the opposite side 19 aof the structural member that locally refines the grain structure of theunmixed portion without the probe breaching the opposite side 19 a.Preferably, to refine the grain structure of the unmixed portion 21 ofthe structural member 11 between the probe tip 18 a and the oppositeside 19 a of the structural member, the probe 18 is thrust into thestructural member to a depth D such that the probe tip is a distance dof approximately 0.007 inches from the opposite side of the structuralmember.

According to another embodiment of the present invention, as illustratedin FIGS. 5A and 5C, the entire thickness of the structural member 11 canbe mixed to define a region 16 of locally refined grain structure(referred to herein as “fill-penetration mixing”). Full-penetrationmixing is preferred for relatively thin structural members, such asstructural members with thickness less than approximately 0.25 inches,but can also be employed to form regions 16 of locally refined grainstructure in structural members with thickness greater than 0.25 inches.As illustrated in FIG. 5A, to full-penetration mix a structural member11 with thickness less than approximately 1.5 inches, the rotating probe18 can be inserted into the outer surface 19 of the structural memberand thrust through the entire thickness of the structural member suchthat the probe breaches the opposite side 19 a of the structural member.After forming the region 16 of locally refined grain structure in thestructural member, both sides 19, 19 a of the structural member can bemachined to provide a finished surface.

When forming elongate regions 16 of locally refined grain structure, asdiscussed below, the rate of travel of the probe 18 through thestructural member is dependent, in part, upon the thickness of thestructural member 11. Typically, the rate of travel of the rotatingprobe through the structural member is proportional to the thickness ofthe member and ranges from approximately 5 to 30 inches per minute. Forstructural members with thickness greater than approximately 1.5 inchesand, particularly, for structural members with thickness greater thanapproximately 3 inches full-penetration mixing of the structural memberis preferably effected by partial-penetration mixing the structuralmember from both sides 19, 19 a to thereby allow an increased rate oftravel of the rotating probe through the structural member and to avoidbreaking or damaging the rotating probe. As illustrated in FIG. 5C, astructural member with thickness greater than approximately 1.5 inchescan be full-penetration mixed by inserting the rotating probe 18 intoand through a first outer side 19 of the structural member 11 to a depthα equal to a pre-selected portion of the thickness of the structuralmember to form a first region 36 of locally refined grain structure. Arotating probe 18 can then be inserted into and through the oppositeside 19 a of the structural member 11 opposite to the first locallyrefined region 36 to a depth β to form a second region 36 a of locallyrefined grain structure. In one embodiment, the depth β is approximatelyequal to the thickness of the structural member 11 less the insertiondepth α of the probe in the first outer side 19 of the structuralmember. In another embodiment, the depth β is greater than the thicknessof the structural member 11 less the insertion depth α of the probe inthe first outer side 19 such that the second region 36 a of locallyrefined grain structure at least partially overlaps the first region 36of locally refined grain structure.

For structural members 11 having elongate regions of comparatively highoperational stress, the rotating friction stir welding probe 18 can bemoved through the structural member 11 along a path corresponding to theregion of high operational stress, as illustrated by the arrow 30 inFIG. 6, to create an elongate region 16 of locally refined grainstructure. According to one embodiment, as illustrated in FIG. 7A, forstructural members 11 having continuous surface areas that are subjectedto comparatively high operational stresses, a plurality of reinforcing“ribs” 26, which are elongate regions 16 of refined grain structure, canbe formed in the structural member using a rotating friction stirwelding probe 18.

As previously discussed and, as illustrated by the photograph of FIG. 8,the regions 16 of locally refined grain structure resist the formationand propagation of micro-cracks in the surface of the structural membersuch that the cracks 33 generally do not intersect or traverse thelocally refined regions, but rather propagate along the periphery of theregions. In one embodiment, as illustrated in FIG. 7B, reinforcing ribs26 can be formed about the surface of the structural member 11 so as tointersect other ribs to thereby define bounded regions or containmentzones 32. Advantageously, the intersecting ribs 26 restrict thepropagation of micro-cracks 33 formed in the surface of the structuralmember to the area defined by the corresponding containment zone, thus,significantly improving the fatigue resistance of the structural memberin the region of comparatively high operational stress. For structuralmembers 11 with thickness of approximately 0.5 inches or less,preferably the reinforcing ribs 26 defining the containment zones 32 areformed by fall-penetration mixing. For relatively thick structuralmembers, the reinforcing ribs 26 defining the containment zones 32 canbe formed by partial-penetration mixing. While the containment zones 32illustrated in FIG. 7B have rectangular configurations and are closed orcompletely bounded by reinforcing ribs 26, the containment zonesaccording to the present invention can also be at least partially openor unbounded and can have other configurations, including both linearand curvilinear configurations. For example, as illustrated in FIG. 7C,an elongate region 16 of locally refined grain structure can be formedhaving a curvilinear portion 39, such as a diminishing spiral, thatconfines and redirects a propagating crack onto itself to blunt thecrack and prevent further propagation.

According to another embodiment of the present invention, as illustratedin FIGS. 9A and 9B, for structural members 11 having continuous surfaceareas that are subjected to comparatively high operational stresses,overlapping regions 16 of locally refined grain structure can be formedto define a continuous area 46 of locally refined grain structure. Asillustrated in FIGS. 9C and 9D, the structural member 11 can then bemachined to remove any excess material 31 to provide a structural memberhaving the desired dimensions and configuration. Advantageously, wherethe structural member 11 includes a flange or other protuberance 34 thatwill be subjected to comparatively high operational stress, such as theone illustrated in FIG. 9C, a continuous area 46 of locally refinedgrain structure can be machined to provide a protuberance 34 havingselectively improved strength, toughness and fatigue resistance.

In another embodiment, the outer surface 19 of the structural member 11defines a notch, groove, aperture or other surface discontinuity 35,which concentrates stress resulting in comparatively high operationalstress proximate to the discontinuity. For example, as illustrated inFIG. 9C, the structural member 11 can include a sharp machined radii 38where a protuberance 34 interconnects with the member; as illustrated inFIG. 11, the structural member can include a threaded opening 37 tofacilitate securing the member to other structural members to form astructural assembly 10; or, as illustrated in FIG. 11, the structuralmember can include an aperture that defines a window or opening 40 in anaerospace vehicle. Prior to machining or forming a threaded opening 37or other stress raising discontinuity 35, an area of the structuralmember 11 that at least partially encompasses the discontinuity can bemixed with a rotating friction stir welding probe 18 to form a region 16of locally refined grain structure. The threaded opening 37 or otherdiscontinuity 35 can then be machined into the outer surface 19 of thestructural member 11 such that the discontinuity 35 is at leastpartially contained within the region 16 of locally refined grainstructure. As illustrated in FIG. 10, the region 16 of locally refinedgrain structure preferably encompasses and surrounds the threadedopening and extends away from the centerline of the opening a distanceranging from approximately the diameter of the threaded opening to twicethe opening diameter. The enhanced material properties of the mixedregion 16 will compensate for the increased operational stress in theimmediate vicinity of the discontinuity 35.

In one embodiment, a threaded opening 37 is formed by mixing the area ofthe structural member 11 that encompasses the discontinuity 35 with arotating friction stir welding probe 18 to form a region 16 of locallyrefined grain structure. The rotating probe 18, which preferably hasthreads with dimensions corresponding to the threads of the threadedopening 37, is moved through the structural member 11 to the location onthe outer surface 19 of the structural member where the threaded openingis to be formed and is inserted into the member to a depth correspondingto the desired depth of the threaded opening. Once the rotating probe 18is in the desired location and depth, rotation of the probe isdiscontinued. The newly formed region 16 of locally refined grainstructure is then allowed to cool and, thereafter, the probe 18 iswithdrawn from the structural member 11 by unthreading the probe fromthe structural member to thereby define the threaded opening 37.Advantageously, the threads of the threaded opening 37 are encompassedby the region 16 of locally refined grain structure so that the threadswill have enhanced material properties to compensate for increasedoperational stress.

In another embodiment, as illustrated in FIGS. 3A and 3B, the endportions 55 a, b of a cast I-beam 57 can be mixed with a rotatingfriction stir welding probe 18 prior to final machining to form elongateregions 16 of locally refined grain structure to compensate for theincreased operational stress in the immediate vicinity of the sharpmachined radii 58 adjacent where the web interconnects with the endportions. The elongate regions 16 of locally refined grain structurepreferably overlap to define a continuous area 46 of locally refinedgrain structure that extends through the end portions 55 a, b of theI-beam and at least partially into the corresponding ends of the web 54.After forming the elongate regions 16 of locally refined grainstructure, the I-beam 57 can be machined to remove excess material 53 toprovide a structural member 11 having the desired dimensions andconfiguration.

In another embodiment, as illustrated in FIGS. 11, a window or opening40 in a structural assembly (not shown), such as an aerospace vehicle,is formed by casting a structural member 11 in a pre-selectedconfiguration having the desired opening, as is known in the art. Priorto final machining, the structural member 11 is mixed with a rotatingfriction stir welding probe 18 about at least a portion of thecircumference of the window 40 to form a region or regions 16 of locallyrefined grain structure that have enhanced material properties tocompensate for increased operational stress in the immediate vicinity ofthe discontinuity 35. While the locally refined region or regions 16 canbe formed by partial-penetration mixing, preferably, the structuralmember 11 is fall-penetration mixed about the circumference of thewindow 40 to form a plurality of overlapping elongate regions 16 oflocally refined grain structure. Overlapping regions 16 of locallyrefined grain structure can also be formed on either end of thestructural member 11 to define continuous areas 46 of locally refinedgrain structure that at least partially encompass a plurality ofthreaded openings 37 for securing the structural member to otherstructural members to form the structural assembly. After forming theregion or regions 16 of locally refined grain structure, the structuralmember 11 can be machined to remove excess material 53 to provide astructural member having the desired dimensions and configuration.

Referring to FIG. 12, there is illustrated a cast tubular structuralmember 11, according to one embodiment of the present invention, thatwill be subjected to a torque load 42. The torque load will result inthe structural member 11 being subjected to comparatively highermagnitudes of operational shear stress such that the expected failuremode is a 45° helically shaped shear failure zone. To compensate forincreased operational stress, the structural member is preferably mixedwith a rotating friction stir welding probe 18 to define one or moreelongate regions 16 of locally refined grain structure having a spiralconfiguration. Advantageously, while the unmixed portions 43 of the caststructural member 11 are relatively brittle, the locally refined regions16 are relatively ductile and, thus, provide zones for yielding therebyimproving the strength, toughness and fatigue resistance of thestructural member 11 in the corresponding regions of comparatively highoperational stress.

In another embodiment, as illustrated in FIGS. 13A and 13B, a caststructural member 11 is subjected to machining during fabrication, suchas stretching a portion of the structural member to shape the memberinto a desired configuration. Prior to performing the machiningoperation, an area of the structural member 11 that at least partiallyencompasses the area that will be machined can be mixed with a rotatingfriction stir welding probe 18 to form a region 16 of locally refinedgrain structure that has improved ductility and formability relative tothe unmixed portions of the cast structural member. The structuralmember can then be machined into the desired configuration, as is knownin the art. Advantageously, as illustrated in FIG. 13B, when themachining operation is performed, the stretching will occur in thelocally refined region 16 such that any details cast into the unmixedportions 43 of the structural member 11 adjacent the locally refinedregion will remain dimensionally stable throughout the machiningoperation.

According to another embodiment of the present invention, the structuralmember can include one or more inserts joined to the member through aweld joint formed by either a fusion or non-fusion welding process. Forexample, as illustrated in FIGS. 14A-14C, the structural member 51includes an insert 51 a joined to the member through a friction stirweld joint 52. Referring to FIG. 14A, the structural member 51 caninclude a milled recess or aperture 50 having dimensions correspondingto the dimensions of the insert 51 a such that the insert can be slip orpress fit to the structural member prior to welding. While the insert 51a can comprise the same material as the structural member 51,preferably, the insert comprises a different material. Similarly, theinsert and structural member can be formed from the same or a differentfabrication process, such as casting or as a wrought or machinedcomponent. In another embodiment, as illustrated in FIG. 15, the insert51 b can comprise a lining for an aperture 56 defined by the structuralmember 51 and wherein the insert is joined to the structural member by afusion weld joint 52. The transition in grain size and structure betweenthe insert 51 b and the structural member 51 at the weld joint 52,particularly where the insert is formed of a different material or adifferent fabrication process, creates stress risers resulting incomparatively high operational stress. To compensate for increasedoperational stress due to grain size discontinuity, the structuralmember 51 and insert 51 b are preferably mixed with a rotating frictionstir welding probe 18 adjacent to, and along the path of, the weld joint52 to define one or more elongate regions 16 of locally refined grainstructure.

According to another embodiment (not shown), the structural memberdefines an external or internal defect that concentrates stressresulting in comparatively high operational stress proximate to thedefect. For example, an external defect in a cast structural member mayinclude, gas or blow holes communicating with the surface; inclusions,such as scale or oxides; or hot tears and cracks due to shrinkage aftercasting. Internal defects in castings may include internal shrinkage. Anexternal defect in a forging may include laps, laminations, slivers,scabs, seams, bark, or cracks. To compensate for increased operationalstress and heal the defect, the structural member is preferably mixedwith a rotating friction stir welding probe to define one or moreregions of locally refined grain structure.

Once a region 16 of locally refined grain structure having a desiredshape and length is formed in the structural member 11, the rotatingprobe 18 is withdrawn from the member. The withdrawal of the rotatingprobe 18 can result in an irregularity in the outer surface 19 of thestructural member 11. In one embodiment (not shown), the portions of thestructural member containing any irregularities caused by the withdrawalof the rotating probe 18 can be cutaway or filled. Preferably, thestructural member 11 is then machined into a pre-selected shape andthickness, as required by the specific design loads and specificationsof the resulting structural assembly 10, or to obtain the desiredsurface finish. For example, a CNC milling machine can be used tomachine the structural member 11 as necessary.

The rotation of the friction stir welding probe 18 within the structuralmember 11 generates sufficient heat energy to plasticize the surroundingmaterial thereby creating a severely deformed, but highly refined grainstructure. In addition, the mixing process eliminates voids, thus,increasing the density of the structural member 11 in the mixed regions16. Advantageously, the regions 16 of locally refined grain structurehave significantly enhanced strength, toughness and fatigue resistancein comparison to the unmixed portions of the structural member 11. Sincethe regions 16 of locally refined grain structure encompass all or atleast a portion of the region that is anticipated to undergocomparatively high operational stress, the regions 16 of locally refinedgrain structure allow the resulting region to better withstand the highoperational stress. Due to the enhanced material properties of the mixedregions 16 of the structural members 11, the thickness of the structuralmembers in those areas having mixed regions may be reduced to therebyobtain a reduction in the overall weight of a structural assembly 10constructed according to the present invention.

The structural members 11 may also be precipitation hardened to improvethe material properties of the unmixed portions of the members. This isparticularly advantageous for aluminum alloys. Precipitation hardeningof metal alloys is a process whereby the mechanical properties of themetal alloy are improved by the formation of uniformly dispersedparticles or precipitates of one or more secondary phases within theoriginal phase matrix. As is known in the art, precipitation hardeningrequires that the metal alloy undergo two heat treatment processes, thefirst process being a solution heat treatment and the second processbeing a precipitation heat treatment, both of which are conducted atpredetermined temperature schedules. While precipitation hardening maybe conducted either before or after locally refining the grain structureof the structural members 11, preferably, the precipitation hardeningprocess is conducted after forming the regions 16 of locally refinedgrain structure. When precipitation hardening a structural member afterlocally refining the grain structure of the member, the regions 16 oflocally refined grain structure should be sufficiently heated duringmixing so as not to create an excessive amount of residual stressbetween the locally refined regions and the unmixed portions of thestructural member.

Referring now to FIG. 16, there is illustrated the operations performedto manufacture a structural member according to one embodiment of thepresent invention. The first step includes casting the structural memberin a pre-selected configuration. See block 60. Regions of the structuralmember having comparatively high operational stress are identified, suchas by mathematical analysis or based upon prior experience. See block61. The structural member may be precipitation hardened to initiallyimprove the material properties of the entire member. See block 62.

The structural member is then secured to prevent movement. See block 63.A friction stir welding probe is positioned adjacent a region of thestructural member having a comparatively high operational stress. Seeblock 64. A region of the structural member having a comparatively highoperational stress is then mixed with a rotating friction stir weldingprobe to locally refine the grain structure of the structural memberwithin the region of high operational stress to thereby improve thestrength, toughness and fatigue resistance of the structural member inthe region. See block 65. The mixing step includes inserting a rotatingfriction stir welding probe through the outer surface of the structuralmember proximate to the region of high operational stress to locallyrefine the grain structure of the high stress region. See block 66. Themixing step may also include moving the rotating friction stir weldingprobe through the structural member along a path corresponding to theregion of high operational stress. See block 67. In one preferredembodiment, the securing, positioning and mixing steps are repeated toform more than one region of locally refined grain structure within thestructural member. In another preferred embodiment, the positioning andmixing steps are repeated to form the desired number of regions oflocally refined grain structure within the structural member, forexample, a plurality of reinforcing ribs that are spaced apart andgenerally parallel. In one embodiment, the friction stir welding probeis withdrawn from the outer surface of the structural member to therebydefine a threaded opening at least partially within the region of thestructural member having a locally refined grain structure after theinserting step. See block 68.

The structural member is then machined to a corresponding pre-selectedshape and thickness. See block 69. A threaded opening can be machined atleast partially within the portion of the structural member having alocally refined grain structure. See block 70. The structural member canthen be precipitation hardened. See block 71. The structural member isthen secured to other structural members to form the frame of anaircraft. See block 72.

Thus, the present invention provides an improved structural assembly andassociated method of manufacture in which the assembly is constructedfrom structural members having enhanced strength, toughness and fatigueresistance in those regions subjected to comparatively high operationalstresses. The improved structural assembly will have an increasedoperational life and reliability, as well as require less stock materialwith a corresponding decrease in the overall weight of the assembly. Inaddition, the improved method of manufacture also allows for theemployment of more castings, which are typically less expensive tofabricate than an equivalent wrought or machined component, inconstruction of structural assemblies for the aerospace industry.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

That which is claimed:
 1. A method for selectively improving thestrength, toughness and fatigue resistance of a structural member in aregion of high operational stress, the method comprising: securing thestructural member to prevent movement; identifying a region of thestructural member having a comparatively high operational stress;positioning a friction stir welding probe adjacent the region of highoperational stress; and thereafter, inserting a rotating friction stirwelding probe through the outer surface of the structural member tolocally refine the grain structure of the structural member within theregion of high operational stress to thereby improve the strength,toughness and fatigue resistance of the structural member in the region.2. A method as defined in claim 1, further comprising casting thestructural member in a pre-selected configuration prior to said securingstep.
 3. A method as defined in claim 1 further comprising moving therotating friction stir welding probe through the structural member alonga path corresponding to the region of high operational stress after saidinserting step.
 4. A method as defined in claim 1, further comprisingmachining the structural member to a corresponding pre-selected shapeand thickness after said inserting step.
 5. A method as defined in claim1, further comprising machining a threaded opening at least partiallywithin the region of the structural member having a locally refinedgrain structure after said inserting step.
 6. A method as defined inclaim 1, further comprising withdrawing the friction stir welding probefrom the outer surface of the structural member to thereby define athreaded opening at least partially within the region of the structuralmember having a locally refined grain structure after said insertingstep.
 7. A method as defined in claim 1, further comprising the step ofprecipitation hardening the structural member after said inserting step.8. A method as defined in claim 1, further comprising the step ofattaching the structural member to other structural members to form theframe of an aircraft.
 9. A method for selectively improving thestrength, toughness and fatigue resistance of a structural member in aregion of high operational stress, the method comprising: securing astructural member to prevent movement; identifying a region of thestructural member having a comparatively high operational stress;thereafter, mixing the region of the structural member having acomparatively high operational stress with a rotating friction stirwelding probe to locally refine the grain structure of the structuralmember within the region of high operational stress to thereby improvethe strength, toughness and fatigue resistance of the structural memberin the region.
 10. A method as defined in claim 9, wherein said mixingstep comprises: inserting a rotating friction stir welding probe throughthe outer surface of the structural member to locally refine the grainstructure of the structural member within the region of high operationalstress; and thereafter, moving the rotating friction stir welding probethrough the structural member along a path corresponding to the regionof high operational stress.
 11. A method as defined in claim 9, furthercomprising machining the structural member to a correspondingpre-selected shape and thickness after said mixing step.
 12. A method asdefined in claim 9, further comprising machining a threaded opening atleast partially within the region of the structural member having alocally refined grain structure after said mixing step.
 13. A method asdefined in claim 9, further comprising the step of precipitationhardening the structural member prior to said mixing step.
 14. A methodas defined in claim 9, further comprising the step of attaching thestructural member to other structural members to form the frame of anaircraft.